Heat dissipation of electrical and electronic devices is rather essential, since their performance and/or lifetime may be reduced at elevated temperatures if the generated heat cannot be dissipated efficiently. For example, the lifetime of light emitting diodes (LED) can be extended with a decrease of the junction temperature. In some situations, the lifetime may even be doubled with a decrease of ˜10° C. in the junction temperature. With a continuous increase of power density in these devices, fast heat dissipation becomes more and more important.
Traditionally, metals (e.g., aluminum, copper) are widely used in making heat dissipating articles (e.g., heat sinks), mainly due to their high thermal conductivities. However, the manufacturing processes from raw metals to final products, which include machining, tooling, extrusion, etc., greatly limit their geometries. Designs of high heat dissipation efficiencies, usually involve large surface areas and complex geometries, cannot be realized subjected to their manufacturing processes. Even if they are able to be fabricated, the processes are usually long and complicated. Moreover, metals are also highly electrical conductive. Thus, additional processes and/or parts for electrical insulation (e.g., surface insulation coating, adding insulation spacers) are necessary for their use in electrical devices.
Thermally conductive polymer composites, combining good thermal conductivity (TC) and the ability of being molded into complex geometries, are good alternatives of metals in making heat sinks. They can also be electrical insulated, light weight, processed at much lower temperatures than metals, processed compatible with traditional plastic manufactory, etc.
One important and interesting property of thermally conductive polymer composites is the anisotropy of their thermal conductivity. The thermal conductivity of a thermally conductive composite is anisotropic if the in-plane thermal conductivity of a plate sample via injection molding is larger than the through-plane thermal conductivity.
Thermal conductivity of polymers has been traditionally enhanced by addition of thermally conductive fillers, including graphite, carbon fibers, ceramic or metal particles. The performance of the thermal conducting polymers has been affected by filler purity, crystallinity, particle size, and more importantly, affected by three factors, which are the intrinsic thermal conductivity anisotropy of a single crystal of the filler along different crystal directions, the geometry (or aspect ratio) of the filler particles, and the distribution of the particles in the matrix. By changing the above factors, the thermal conductivity anisotropy of the composite can be tuned. It would be useful in the situations of optimizing heat dissipation performance of an article, but its geometry is fixed or does not have much room to change. For instance, thermal conductive composites with high conductivity can be used to replace metals in some applications, for example, as heat sink of LED lighting.